Improve documentation on how to get a whatsapp token

Problem/Motivation

The documentation explains how to configure the module itself. Mainly, how to add an auth key to use with whatsapp.
But there is nothing about the way to get a whatsapp key.

Steps to reproduce

I followed officials instructions by Meta
https://developers.facebook.com/docs/whatsapp/business-management-api/ge...

I created a business app with whatsapp application and different tokens.
None worked once set in this drupal module.

Proposed resolution

Add documentation on Meta/Whatsapp tokens.